In the art of respiration therapy and apparatus, it is well known to employ respiration systems to provide positive and expiration pressure as a therapy for intensive patient care. The effect of such therapy on the pulmonary system is dramatic and beneficial for a variety of disease entities. A valve which can be used in such respiration systems to control positive and expiratory pressure in commonly referred to as a PEEP valve.
In the medical literature, the PEEP valve is defined as a valve which exhibits threshold resistance. That is to say, a threshold resistor should allow no exhalation flow until exhalation pressure equals a predetermined threshold pressure. Above the threshold pressure, the valve should allow exhalation flow rates of up to 200 to 300 liters per minute without any significant increase in pressure drop across the valve. By significant increases is meant an increment exceeding approximately 10% of the threshold resistance value, for example.
Many valve structures known in the prior art are intended to provide control of inhalation and exhalation flow in respiration systems. In particular, the art includes many examples of PEEP valves whose intended function is to provide the desired exhalation threshold resistance as above described. For example, U.S. Pat. No. 2,954,793 discloses an inhalation/exhalation valve which includes a flexible disk inhalation valve element which is carried on an exhalation valve member to accommodate one way inhalation flow therethrough. The patent also discloses an hourglass shaped spring which biases the exhalation valve closed.
Other documents disclosing spring biased exhalation valve elements, and specifically conical springs for biasing the valve element, include U.S. Pats. Nos. 4,411,285; 2,863,446; 3,088,477; 3,630,197 and 3,276,462.
Among other patent art known to me, U.S. Pat. No. 4,574,797 discloses a second stage regulator for an underwater breathing apparatus which includes a flexible flap-type valve member. U.S. Pat. No. 2,820,469 discloses another combined inhalation/exhalation valve. U.S. Pat. No. 3,799,185 discloses yet another such valve, as does U.S. Pat. No. 4,207,884. Both of these patents appear to disclose a flange or skirt formed on the perimeter of a valve disk element. Other patents pertaining generally to breathing gas flow control include numbers U.S. Pat. Nos. 4,446,859; 2,947,314 and 4,592,384.
In the prior art the desirable performance criteria of a threshold resistor have required that very soft springs be used to preload a valve disk which covers a port that is exposed to the controlled pressure. Such devices generally require a very soft and long-loading spring which has a high ratio of preload deformation to displacement of the valve disk during exhalation flow through the valve. In this manner, the designer is better able to predict the force increment required to allow the disk to open a sufficient distance for the desired exhalation flow rates to occur. However, when one uses such a long, soft spring in a PEEP valve, or even the conventional conical springs, adjustability of the valve threshold pressure over a range useful for clinical application (i.e., approximately 3 to 25 cm H.sub.2 O) becomes difficult because the preload deformation of the spring has to be relatively large to achieve the desired range of pressure variation. A shorter conventional spring will tend to be too stiff for use in the range of low operation pressures specified. Preferably, the spring should be very soft at initial valve opening and throughout the desired pressure range, while becoming progressively stiffer in a non-linear manner with incremental opening of the exhalation valve. This can provide for a readily adjustable threshold resistance, and will also permit the valve to reliably maintain the selected threshold resistance throughout the exhalation cycle and for a relatively wide range of exhalation flow rates.
As has been mentioned, it is important that a PEEP valve not only be readily adjustable with a high degree of precision, additionally it should exhibit the performance criterion of pressure or threshold resistance stability. That is, whatever the threshold resistance setting to which the valve has been adjusted, that threshold resistance should be maintained throughout the normal range of exhalation flow rates in order to ensure that a constant, positive end expiratory pressure value will be maintained regardless of the patient's expiratory flow rate.
Although the prior art is replete with valves which are intended to function effectively as PEEP valves, practitioners in the art continue to seek improvements in valve adjustability and operating stability.